אָהַ֣בְתָּ צֶּדֶק֮ וַתִּשְׂנָ֫א רֶ֥שַׁע עַל־כֵּ֤ן ׀ מְשָׁחֲךָ֡ אֱלֹהִ֣ים אֱ֭לֹהֶיךָ שֶׁ֥מֶן שָׂשׂ֗וֹן מֵֽחֲבֵרֶֽיךָ׃
Rashi on Psalms
anointed you...with oil of joy Every expression of greatness is depicted by the anointment of oil, as is the custom of the kings.